Epoxy resin compositions and silicone resin compositions have heretofore been widely used to encapsulate light-emitting semiconductor chips. In general, resin compositions of these types are poured into molds in which light-emitting semiconductor chips are arranged by casting, transfer molding or the like, and are then cured to encapsulate the light-emitting semiconductor chips.
Keeping in step with the adoption of LEDs with higher brightness and higher power in recent years, the problem of discoloration and degradation of epoxy resins has arisen. In particular, transparent epoxy resins undergo yellowing by blue light and ultraviolet rays, and therefore, are accompanied by the problem that they reduce the service life of chips.
Proposed in JP-A 2008-10591 (Patent Document 1) is a light-emitting device capable of avoiding sulfidation of a silver-plated surface by coating the silver-plated surface with a thin film of a resin having low H2S transmission property. An organic resin, however, involves a problem in that it is weak against light and heat and is inferior in long-term reliability. Proposed in JP-A 2009-33107 (Patent Document 2), on the other hand, is a light-emitting device with a metallic reflection member coated with a film of a glass that contains Si—N bonds as an essential feature. However, the glass with Si—N bonds contained therein has not been allowed to react completely, and therefore is unstable. As a result of changes with time through hydrolysis or the like, the glass film is lowered in flexibility to form microcracks, leading to problems that damages to the package may be caused or the gas barrier properties of the glass film may be lowered.
With a view to overcoming the above-described problems, silicone resins excellent in both heat resistance and light resistance have been finding utility. These silicone resins are, however, accompanied by problems that in cured forms, they have higher gas transmission property and lower strength than epoxy resins. Accordingly, those making use of high-hardness rubbery silicone resins for encapsulating purpose have been proposed (JP-A 2002-314139 (Patent Document 3) and JP-A 2002-314143 (Patent Document 4)).
However, these high-hardness rubbery silicone resins are low in adhesive properties, and therefore, involve a problem in that, in encased light-emitting semiconductor devices, i.e., devices with light-emitting chips being arranged in ceramic and/or plastic casings and with the casings being internally filled up with the silicone resins, the silicone resins separate from the ceramics or plastics of the casings in thermal shock tests at from −40 to 120° C.
Even with such high-hardness rubbery silicone resins, the gas transmission property is higher compared with that of epoxy resins and the like, so that corrosive gas such as nitrogen oxides and sulfur oxides transmits through the silicone resins and reacts with the silver-plated parts of reflector surfaces. As a consequence, such high-hardness rubbery silicone resins become a cause of a reduction in photoreflectance, and hence, a reduction in brightness.
In the meantime, an epoxy-containing silicone resin has been proposed to provide increased thermal shock resistance (JP-A H7-97433 (Patent Document 5)). However, this silicone resin is synthesized by subjecting an epoxy-containing silane and a silanol to condensation, and its cured product is low in modulus of elasticity and is brittle. Therefore, an LED molded in a resin of this type is accompanied by a problem that the resin is prone to cracking in a temperature cycling test.
As solutions to the above-described problem, there are known a composition that contains an epoxy resin and a silsesquioxane containing at least two epoxy rings (JP-A 2005-263869 (Patent Document 6)) and a composition that contains an epoxy resin and a silicone resin containing groups of an isocyanuric acid derivative (JP-A 2004-99751 (Patent Document 7)). Neither of these composition are, however, considered to exhibit any satisfactory anti-cracking properties in temperature cycling tests of their cured products.